In recent years, AV equipment or electronic equipment including personal computers and portable communication devices has been pushed toward portable and cordless configurations at high speed. For a driving power source of these electric devices, nickel cadmium storage batteries, nickel metal-hydride storage batteries, lithium secondary batteries, and the like have been used since they have high reliability and easy maintainability. Meanwhile, lead-acid storage batteries are in the current mainstream of such applications as an emergency backup power source intended for power failures occurring from disasters like an earthquake and typhoon, whereas the use of nickel metal-hydride storage batteries having high capacity, capable of a high-current discharge, is expected in the future. Nickel metal-hydride storage batteries having high capacity are also expected to be adopted for an emergency power source in unattended communication stations or the like and for railway power applications such as a power source for moving a train pantograph up and down and a lighting backup power source to be used when a railway power feed fails.
Power sources as mentioned above are typically assembled in the form of a battery module or a battery pack by connecting the opposite power terminals of a plurality of cylindrical batteries to each other. The cell-to-cell connection structures to be used for constituting this battery module or battery pack are broadly classified into two, depending on the cell arrangement. One is to arrange a plurality of batteries Ba1 and Ba2 along the axial direction as shown in FIG. 8, and connect each adjoining pair of batteries Ba1 and Ba2 in series (for example, see Patent Document 1). In this cell-to-cell connection structure, a bowl-shaped connector 50 interposed between two batteries Ba1 and Ba2 is connected to both a sealing plate 52 having a metal cap 51 formed thereon, as a positive electrode terminal of one battery Ba1, and a battery case, as a negative electrode terminal of the other battery Ba by projection welding respectively.
More specifically, the connector 50 comprises a cylindrical part 50a to be fit onto a battery case 53, and a flat part 50b to make contact with a sealing plate 52. Projections 50c are formed on the inner surface of the cylindrical part 50a and the outer surface of the flat part 50b in respective locations on the same radii at regular intervals of 90°. By projection welding via the foregoing projections 50c, the flat part 50b of the connector 50 is connected to the sealing plate 52 that is electrically connected with the metal cap 51 of the one battery Ba1, and the cylindrical part 50a is connected to the battery case 53 of the other battery Ba2. An electric short circuit between the battery case 53 of the one battery Ba1 and the connector 50 is precluded by an insulation ring. The two batteries Ba1 and Ba2 are thus electrically connected in series.
The other cell-to-cell connection structure is to juxtapose a plurality of batteries Ba3 and Ba4 in a radial direction and establish a series connection by using a connecting member 59 as shown in FIG. 9 (for example, see Patent Document 2). In this cell-to-cell connection structure, the outer surface of a positive electrode terminal 60 at an end face of one battery Ba3 and an exposed bottom 55a, at the other end face, of a battery case 55 of the other battery Ba4 are juxtaposed so that they are flush with each other while the peripheries are in close contact with each other. In this state, the connecting member 59 is interposed across the two, and this connecting member 59 and the batteries Ba3 and Ba4 are welded for electric connection. More specifically, a first connecting part 61 of this connecting member 59 is welded to the bottom 55a of the battery case 55, and an annular recessed portion of a second connecting part 62 thereof is welded to a sealing plate 64.    [Patent Document 1] Japanese Patent Laid-Open Publication No. 2003-162993    [Patent Document 2] Japanese Patent Laid-Open Publication No. 2002-246005